ELI5 Eczema - why scratching it feels so good?

Why does scratching eczema affected skin feel good in that moment, when it actually worsens the skin condition? Why is it that eczema often appears in small patches on the body rather than being spread out? How does hydrocortisone work to relieve itching and inflammation caused by eczema? Is Eczema contagious?
... we explain like I'm five
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